Fibre-reinforced compositions and methods for producing such compositions

ABSTRACT

Fibre-reinforced structures comprising a thermoplastics polymer and containing at least 30% by volume of reinforcing filaments extending longitudinally of the structure which have been produced in a continuous process and which have exceptionally high stiffness. The exceptionally high stiffness results from thorough wetting of the reinforcing filaments by molten polymer in the continuous process. The thorough wetting gives rise to a product which can be further processed even in vigorous mixing processes such as injection moulding with surprisingly high retention of the fibre length in the fabricated article. The continuous processes for producing the reinforced structures employ thermoplastics polymers having lower melt viscosities than conventionally considered suitable for achieving satisfactory physical properties.

This is a continuation of application Ser. No. 07/639,341, filed Jan.10, 1991, now abandoned, and a continuation of application Ser. No.07/401,488, filed Aug. 30, 1989, now U.S. Pat. No. 5,019,450, which is acontinuation of Ser. No. 07/133,237, filed Dec. 14, 1987, now abandoned,which is a continuation of Ser. No. 06/804,114, filed Dec. 3, 1985, nowabandoned, which is a division of Ser. No. 06/537,713, filed Sep. 30,1983, now U.S. Pat. No. 4,559,262, which is a continuation of Ser. No.06/341,186, filed Jan. 20, 1982, now abandoned.

This invention relates to fibre-reinforced compositions containingthermoplastics resins and to methods of producing such compositions.

Processes are known in which a fibre-reinforced structure is produced bypulling a tow or roving of glass fibres through a bath of low viscositythermosettable resin to impregnate the fibres. The structure issubsequently cured by heating. Such processes are known as pultrusionprocesses. Although such processes have been known for at least 10 yearsthey have not been used commercially to any extent for the production ofthermoplastics resin impregnated structures. This is because ofdifficulties experienced in wetting the fibres when they are pulledthrough the viscous molten resin. The resulting products haveunacceptable properties as a result of this poor wetting.

The efficiency of a particular process in wetting the fibres and therebyproviding the basis for making maximum use of the very high levels ofphysical properties inherent in continuous fibres, such as glass fibres,may be assessed by measuring the extent to which the process provides aproduct having a flexural modulus approaching the theoreticallyattainable flexural modulus.

The theoretically attainable flexural modulus is calculated using thesimple rule of mixtures:

    E.sub.L =V.sub.f E.sub.f +V.sub.m E.sub.m

where

E_(L) is the longitudinal modulus of the composition,

V_(f) is the volume fraction of fibre,

E_(f) is the flexural modulus of the fibre,

V_(m) is the volume fraction of the matrix polymer,

E_(m) is the flexural modulus of the matrix polymer.

The use of melts of conventional high molecular weight thermoplasticsfor impregnating continuous rovings does not permit a high level offlexural modulus to be obtained. For example, U.S. Pat. No. 3,993,726discloses an improved process of impregnating continuous rovings underhigh pressure in a cross-head extruder, pulling the rovings through adie and cooling and shaping the rovings into a void-free shaped article.Products obtained using polypropylene are shown in Example 1 to have aflexural modulus of only about 6 GN/m², for a glass fibre content of 73%by weight, that is, less than 20% of the theoretically attainable value.

It has now been found possible to produce materials having flexuralmodulus levels which approach the theoretically attainable levels.

Accordingly, there is provided a fibre-reinforced structure comprising athermoplastics polymer and reinforcing filaments characterised in thatthe structure has been produced by a continuous process and contains atleast 30% by volume of the structure of reinforcing filaments extendinglongitudinally of the structure and the flexural modulus of thestructure determined by ASTM D790-80 is at least 70% and preferably atleast 90% of the theoretically attainable flexural modulus. Theinterlaminar shear strength of these structure is in excess of 10 MN/m²and preferably in excess of 20 MN/m². The preferred thermoplasticpolymers for use in the invention are crystalline polymers having amelting point of at least 150° C. and amorphous polymers having a glasstransition temperature of at least 25° C. For optimum stiffness thethermoplastic polymer should have a flexural modulus of at least 1 GN/m²and preferably at least 1.5 GN/m².

Fibre reinforced structures according to the invention can be producedby a variety of processes which enable good wetting of continuous,aligned, fibres to be achieved. In one of these processes there isprovided a process of producing a fibre-reinforced compositioncomprising drawing a plurality of continuous filaments through a melt ofa thermoplastics polymer having a melt viscosity of less than 30 Ns/m²,preferably between 1 and 10 Ns/m², to wet the filaments with moltenpolymer, the filaments being aligned along the direction of draw.Optionally, the impregnated filaments may be consolidated into afibre-reinforced polymer structure. The viscosity of thermoplasticsvaries with shear rate decreasing from a near constant value at lowshear. In this application we refer to the viscosity at low shear rates(usually referred to as the Newtonian viscosity). This is convenientlymeasured in a capillary viscometer using a die 1 mm in diameter and 8 mmlong, the melt viscosity being determined at a shear stress in the range10³ -10⁴ N/m².

Surprisingly, despite the fact that such a polymer is of lower molecularweight than normally considered suitable in the thermoplastics polymerfield for achieving satisfactory physical properties the reinforcedcompositions have exceptionally good physical properties. Thus whenreinforced thermosetting polymer compositions are produced by apultrusion process the viscosity of the thermosetting prepolymer resinin the impregnation bath is typically less than 1 Ns/m² for good wettingof the fibre. This low value of viscosity may be used because theprepolymer is subsequently converted to a solid form by a heat curingprocess. By contrast thermoplastics polymers are normally fullypolymerised solid materials and are only obtained in liquid form byheating the thermoplastics polymer to melt it. However, the meltviscosity of conventional high molecular weight polymers havingacceptable physical properties is usually in excess of 100 Ns/m².Adequate wetting of the fibres in a pultrusion process with a melt ofsuch high viscosity is not possible. Whilst it is possible to reducemelt viscosity to some extent by increasing the melt temperature it isnormally the case that an insufficient reduction in viscosity isobtainable below the decomposition temperature of the thermoplasticspolymer.

The use of a thermoplastics polymer of low enough molecular weight togive a sufficiently low melt viscosity to result in adequate fibrewetting in a pultrusion process surprisingly gives a product of highstrength.

Accordingly there is also provided a fibre-reinforced thermoplasticscomposition characterised in that it has been obtained by drawing aplurality of continuous filaments through a melt of the thermoplasticspolymer having a melt viscosity of less than 30 Ns/m² and preferablybetween 1 and 10 Ns/m² to wet the filaments with molten polymer, thefilaments being aligned in the direction of draw. The fibre-reinforcedstructure produced should have a void content of less than 15% andpreferably less than 5%.

By the term "continuous fibres" or "plurality of continuous filaments"is meant any fibrous product in which the fibres are sufficiently longto give a roving or tow of sufficient strength, under the processingconditions used, to be hauled through the molten polymer without thefrequency of breakage which would render the process unworkable.Suitable materials are glass fibre, carbon fibre, jute and high modulussynthetic polymer fibres. In the latter case it is important that thepolymer fibres conform to the proviso of having sufficient strength tobe capable of being hauled through the polymer melt without breakagedisrupting the process. In order to have sufficient strength to behauled through the impregnation system without breakage the majority ofthe continuous fibres of the fibrous product should lie in one directionso that the fibrous product can be drawn through molten polymer with themajority of the continuous fibres aligned. Fibrous products such as matsmade up of randomly disposed continuous fibrous are not suitable for usein the invention unless they form part of a fibre structure in which atleast 50% by volume of the fibres are aligned in the direction of draw.

The continuous fibres may be in any form having sufficient integrity tobe pulled through the molten polymer but conveniently consist of bundlesof individual fibres or filaments, hereinafter termed "rovings" in whichsubstantially all the fibres are aligned along the length of thebundles. Any number of such rovings may be employed. In the case ofcommercially available glass rovings each roving may consist of up to8000 or more continuous glass filaments. Carbon fibre tapes containingup to 6000 or more carbon fibres may be used. Cloths woven from rovingsare also suitable for use in the present invention. The continuousfibres may be provided with any of the conventional surface sizes,particularly those designed to maximise bonding between the fibre andthe matrix polymer.

In order to achieve the high levels of flexural modulus possible by useof the invention it is necessary that as much as possible of thesurfaces of the continuous fibres are wetted by the molten polymer. Thuswhere a fibre consists of a plurality of filaments the surfaces of theindividual filaments making up the fibre must be wetted for optimumeffect. Where the filament is treated with a surface size or anchoringagent the polymer will not be in direct contact with the surface of thefibre or filament because the size is interposed. However, providingthat good adhesion between the fibre and the size and between the sizeand the polymer are achieved the product of the invention will have ahigh flexural modulus and the size will, in general, enhance theproperties obtained.

The thermoplastics polymer employed in the process hereinbeforedescribed may be any polymer which will melt to form a coherent massproviding that the melt has a viscosity of less than 30 Ns/m² andpreferably less than 10 Ns/m². In order to achieve acceptable physicalproperties in the reinforced composition it is preferred that the meltviscosity should be in excess of 1 Ns/m². As indicated the choice of apolymer in the required melt viscosity range is primarily dictated bythe molecular weight of the polymer. Suitable polymers includethermoplastics polyesters, polyamides, polysulphones, polyoxymethylenes,polypropylene, polyarylene sulphides, polyphenylene oxide/polystyreneblends, polyetheretherketones and polyetherketones. A variety of otherthermoplastics polymers can be used in the process of the inventionalthough polymers such as polyethylene will not give compositions ofsuch high strength.

In the process of impregnating the fibres of the roving, in addition tousing a polymer of suitable melt viscosity to bring about adequatewetting it is necessary to maximise penetration of the melt into theroving. This may be done by separating, as far as possible, the rovingsinto the individual constituent fibres, for example, by applying anelectrostatic charge to the roving prior to its entry into the moltenpolymer or preferably by spreading the roving whilst it is in the moltenpolymer to separate the constituent filaments. This is convenientlyachieved by passing the roving under tension over at least one, andpreferably several, spreader surfaces. Further enhancement of wettingoccurs if further work is applied to the separated, polymer impregnated,fibres, for example, by consolidating the separated fibres by pullingthe impregnated roving from the melt through a die. This die may havethe profile desired of the impregnated roving or the impregnated rovingmay be passed through a further sizing die whilst the polymer is stillflowable. Surprisingly, it is advantageous if this die is cooled inorder to achieve satisfactory sizing and a smooth passage through thedie. When the impregnated rovings emerge from the bath in the form of aflat sheet further work may be applied by passing the sheet between apair of rollers.

The rate at which the roving can be hauled through the impregnation bathis dependent on the requirement that the individual fibres should beadequately wetted. This will depend, to a large extent on the length ofthe path through the molten polymer bath and in particular the extent ofthe mechanical spreading action to which the roving is subjected in thebath. The rates achievable in the present process are at leastcomparable with the rates achievable in thermoset pultrusion processesbecause the latter process is restricted by the time required tocomplete necessary chemical reactions after the impregnation stage.

In a preferred embodiment the spreader surfaces over which the rovingsare pulled to provide separation of the rovings are provided with anexternal heat input to heat the spreader surface a temperature above themelting point of the particular polymer to be used for impregnating theroving. By this means the melt viscosity of the polymer in the localregion of the spreader surface may be maintained at a considerably lowervalue than the polymer in the bulk of the impregnation bath. Thisprocess has the advantage that a very small proportion of the polymercan be raised to a relatively high temperature so that a lowimpregnation viscosity can be obtained with minimum risk of degradationto the major proportion of the polymer in the bath. This in turn greatlyreduces the problem arising from the fact that some polymer may remainin the bath for an almost unlimited period during a given processingperiod because the polymer supply in the bath is continuallyreplenished. Thus some of the polymer present at the start of aprocessing period may still be present in the bath at the end of theprocessing period. Despite this long dwell time in the bath such polymerwill have been subjected to a less severe thermal history than if thewhole of the polymer in the bath had been subjected to a hightemperature to obtain a low viscosity throughout the bath.

A further advantage of the local heating process is that polymers ofpoorer thermal stability may be used. Furthermore, polymers of highermolecular weight may be used because the lower degradation arising fromthe lower overall thermal history enables a higher temperature to beused locally to produce lower viscosity melt.

The supply of polymer to the impregnation bath may be in the form ofpolymer powder which is melted in the bath by external heating elementsor by the internally disposed heated spreader surfaces or alternativelythe bath may be fed with molten polymer using, for example, aconventional screw extruder. If the bath is provided with heatedspreader surfaces the polymer melt delivered from the extruder should beat as low a temperature as possible to minimise thermal degradation. Theuse of a melt feed has the advantages of easier start-up, bettertemperature control and the avoidance of unmelted polymer lumps whichresult in various processing problems particularly when very thinstructures are produced.

The impregnated fibre product may be pulled through a means forconsolidating the product such as a sizing die. The temperature of thedie has been found to have a significant effect on the process. Althoughit would be predicted that a hot die should be used to minimise frictionin the die and to aid consolidation it has been found that a die whichis held at a temperature at or above the melting point of the polymerused gives rise to an erratic stick-slip behaviour as the product ispulled through the die. It has been found that it is preferable to use acooled die and to ensure that the surface temperature of the pultrudedsection entering the die is at a temperature of not more than 20° C.above the softening temperature of the polymer. By "softeningtemperature" we mean the lowest temperature at which the polymer can besintered. This can be achieved by blowing air on the lace during itspath between the impregnation bath and the die and/or spacing the diefrom the impregnation bath. If the pultruded section is too hot polymeris squeezed out as the product enters the die. This leaves a deposit atthe entrance to the die which builds up and can score the pultrudedsection as it passes through the die. The pultruded section should notbe cooled to a temperature below the softening point of the polymerbecause it will be too difficult to shape the product in the sizing die.

The dimensions of the fibre-reinforced product can be varied as desired.Thin sheet can be produced by separating the fibres of a number ofrovings by passing them over a spreader surface so that the fibres forma band in contiguous relationship. Where a die has been used toconsolidate the fibres the structure will take on the cross section ofthe sizing die. This can give articles of any required thickness, forexample from 0.25 mm to 50 mm thick or linear profiles. Where the meansof consolidation comprises the nip formed from at least one pair ofrotating rollers, sheets having a thickness of 0.05 mm or even less canbe produced.

In a further process for producing fibre-reinforced structures accordingto the invention it has been found possible to achieve satisfactorywetting even though the thermoplastics polymer used has a melt viscositysignificantly in excess of 30 Ns/m².

Accordingly there is provided a process of producing a fibre-reinforcedcomposition comprising tensioning and aligning a plurality of continuousfilaments to provide a band of contiguous filaments, passing the bandover a heated spreader surface so as to form a nip between the band andthe spreader surface, maintaining a feed of a thermoplastics polymer atthe nip, the temperature of the spreader surface being sufficiently highto give a polymer melt of viscosity capable of wetting the continuousfilaments as they are drawn over the spreader surface. Whilst it ispreferred that the polymer melt in the cusp of the nip has a viscosityof less than 30 Ns/m², a high back tension on the filaments fed to thespreader surface will ensure that polymer impregnation in the nip areais favoured, so that it is possible to produce a well impregnated bandat a significantly higher viscosity than 30 Ns/m². Thus this processprovides a means of maximising the molecular weight of the polymer thatmay be used in a thermoplastics polymer pultrusion process.

In one embodiment of this process the continuous filaments are mostsuitably tensioned and aligned by pulling them from rolls or reels overa series of spreader surfaces, such as the surfaces of rods. Thisenables bundles of filaments to be spread out as far as possible intoindividual filaments which are under considerable tension. Thesefilaments are guided to provide a band of contiguous filaments as theypass over a heated spreader surface. The shape of the spreader surfaceand the angle of contact of the filament band with the surface should besuch as to provide a nip between the band and the heated spreadersurface. A thermoplastics polymer powder is fed to the nip and theheated spreader surface is maintained at a temperature sufficient tomelt the thermoplastics polymer. The melt impregnates and wets thefibres of the band as the band passes over the heated spreader surface.

This process may be further modified by providing at least one furtherheated spreader surface with which the at least partially polymerimpregnated fibre band forms a second nip by means of which a furthersupply of polymer melt may be impregnated into the fibre band. Eithersurface of the partially impregnated band can be used to form theworking surface of the nip.

The amount of polymer in the reinforced structure is controlled to alarge extent by the tension in the band and the length of path overwhich the band contacts the heated spreader surface. Thus where the bandis under high tension and contacts the spreader surface over asubstantial area, so that the band is strongly urged against thespreader surface, the polymer content of the reinforced structure willbe lower than under low tension/short contact path conditions.

The heated spreader surfaces and any subsequent heated or cooledsurfaces used to improve impregnation or to improve surface finish arepreferably in the form of cylindrical bars or rollers. These may bestationary or capable of either free or driven rotation. For example,the first impregnation surface may be a freely rotating roller whichwill be caused to rotate by the band at the speed of the band so thatattrition of the fibre prior to impregnation or sizing by the melt isreduced to a minimum. It has been observed that if the first roll isrotated (either freely or driven) in the direction of the movement ofthe fibre at up to the speed of the fibre any accumulation of loosefibre on the band is carried through the system. This self-cleaningaction is particularly useful in preventing an accumulation of fibre atthe first roll which could cause splitting of the band. After the bandhas picked up some molten polymer, preferably after being provided withfurther molten polymer on the other side of the band by means of asecond freely rotatable heated surface, the fibre is much lesssusceptible to attrition and may be subjected to treatments to improvewetting of the fibres. Thus the polymer-containing band may be passedover at least one roller driven in a direction opposite to that of thetravel of the band to increase the local work input on the band andmaximise wetting. In general, the degree of wetting and the speed of theprocess may be increased by increasing the number of surfaces at whichthere is a work input.

A further advantage of the process employing a band of fibre to form anip, over the process which requires the use of a bath of moltenpolymer, is that of reducing the risk of degradation. Thus therelatively small amount of polymer present in the nip between the fibreband and the spreader surface ensures that large quantities of polymerare not held at an elevated temperature for prolonged periods. Provisioncan also be made to include a scraper blade at positions at whichpolymer is fed to the nip to remove any excess polymer which mightaccumulate during processing and which might be subject to thermaldegradation.

When the product of the process of the invention is required as a thinreinforced sheet the product produced by impregnation at a nip may befurther treated by passing over or between further rolls either heatedor cooled to improve impregnation or to improve the surface finish ofthe sheet. The thin sheet has a tendency to curl if one side of thesheet contains more polymer than the other side. This situation can beavoided by positioning an adjustable heated scraper in proximity to thelast roll in the series to remove excess polymer on the sheet surface.The scraper bar should be at a temperature just in excess of the meltingpoint of the polymer. For example in the case of polyetheretherketonewhich reaches a temperature of about 380° C. in the impregnation zonesthe scraper bar temperature should be about 350° C.

The impregnated band may then be subjected to further treatmentsdepending on the intended shape and purpose of the end product. Theseparated filaments in the impregnated band may, for example, be drawntogether through a die to provide a profile of considerably greaterthickness than the impregnated band. A limited amount of shaping may beeffected in such a die to provide a shaped profile.

The impregnated products of the processes hereinbefore described may bewound on rolls for subsequent use in fabrication processes requiring acontinuous product or may be chopped into lengths for subsequentfabrication. The continuous lengths may be used to fabricate articles,for example, by winding the heat-softened product around a former, or,for example, by weaving a mat from tapes or strips of the product. Theimpregnated product may be chopped into pellets or granules in which thealigned fibres have lengths from 3 mm up to 100 mm. These may be used inconventional moulding or extrusion processes.

When glass fibre is used the fibre content of the product of theinvention should be at least 50% by weight of the product to maximisethe physical properties of the product. The upper limit of fibre contentis determined by the amount of polymer required to wet out theindividual fibres of the roving. In general it is difficult to achievegood wetting with less than 20% by weight of polymer although excellentresults are obtainable using the process of the invention to incorporate30% by weight of polymer in the fibre-reinforced composition.

The product of the invention formed by the process of impregnating aband of contiguous rovings at a nip formed by the band and a heatedspreader surface will normally be hauled off the impregnation system asa band or sheet of material. This provides a useful intermediate formany applications. Thin bands or sheets, that is those having athickness of less than 0.5 mm and greater than 0.05 mm are particularlyuseful and versatile.

Tapes are particularly useful for forming articles woven using a tabbyor satin weave (these terms are used in the weaving art and aredescribed in the Encyclopedia Brittanica article on "Weaving"). A satinweave gives a particularly good product as shown in the examples of thisspecification. Woven articles of exceptionally high performance areobtained using the tapes produced according to this invention and havinga breadth of tape at least 10 times the thickness of the tape. Animportant application is as a thin reinforced sheet which is to be usedto form a reinforced article from a number of plies of the reinforcedsheet, with the reinforcement of each layer disposed in any chosendirection in the plane of the layers, by compressing the layers at atemperature sufficient to cause the polymer of the layers to coalesce.The layers may be used as flat sheets which may be shaped in a mouldduring or after the coalescing stage, or they may be wound or formed ona shaped mandrel which after a coalescing stage gives an article havingthe shape of the mandrel.

It is already known, for example as disclosed in British PatentSpecification No. 1 485 586 to produce reinforced shaped articles bywinding reinforcing filaments on a shaped mandrel and interposing layersof polymer film between layers of filaments with subsequent coalescenceof the polymer films. The present invention has advantages over such aprocess. The major benefits are the avoidance of the use of high costperformed polymer films, the avoidance of the need to provide films ofvarious thicknesses because the polymer content can be controlled by thetension in the band and the benefits derived from the continuous natureof the process of the invention.

The pultruded products of the invention are also suitable, when choppedto appropriate dimensions for providing selective reinforcement inshaped articles moulded from polymeric materials in a process in whichat least one preformed element consisting of a product according to thepresent invention is located in a mould to provide reinforcement in aselected portion of the finished moulding and polymeric material ismoulded around the in situ reinforcement to provide a shaped article.

The invention not only permits the fibrous reinforcement to be locatedin the shaped article with maximum effect with respect to the stressesthe shaped article will be subjected to in use but overcomes theprocessing problems involved in producing such high strength articles byalternative processes. In particular the process can be used to producesuch reinforced articles using the high productivity injection mouldingprocess using conventional thermoplastic polymers with melt viscositiesof 100 Ns/m² or more.

It may be advantageous in some applications to use the performed elementat a temperature at which it is readily pliable so that it can be morereadily located in the mould, for example by winding the heat softenedperformed element on a mould insert.

The moulding process used may be any process in which a shaped articleis formed from a polymeric material in a mould. The polymeric materialmay be a thermoplastics material which is introduced into the mould as amelt, such as injection moulding or as a powder, as in compressionmoulding. Included in the term "compression moulding" is the process ofcompressing a polymer powder without melting and subsequently sinteringthe "green" moulding outside the mould. The thermoplastics polymericmaterial moulded in the mould may also be derived by introducing intothe mould a monomer or monomers, or a partially polymerised medium whichis held in the mould until fully polymerised, for example, under theinfluence of heat or chemical activators or initiators.

It is preferred that the polymer which is moulded around the preformedinsert is the same as or is at least compatible with the polymer used toimpregnate the preformed insert.

The impregnated products obtained from the processes hereinbeforedescribed find particular utility when chopped into pellets or granulesin which the reinforcing fibres have a length of at least 3 mm andpreferably at least 10 mm. These products may be used in theconventional fabrication process such as injection moulding and showadvantages over prior art products in pellet form because the fibrelength in the pellet is retained to a much greater extent in articlesfabricated from the pellets of the invention than when using the priorart products. This greater retention of fibre length is believed to be aresult of the greater protection afforded to the individual reinforcingfilaments in the product of the invention by virtue of the good wettingby polymer which arises from use of the processes hereinbeforedescribed.

This aspect of the invention is particularly important because itenables reinforced articles to be formed in versatile operations, suchas injection moulding, which employ screw extrusion processes to meltand homogenise the feed material, with a surprisingly high retention offibre length and consequent enhancement of physical properties. Thus theproduct of the invention enables moulded articles to be obtained fromfabrication processes which employ screw extrusion which articlescontain at least 50% and preferably at least 70% by weight of the fibresin the article of a length at least 3 mm long. This is considerablylonger than currently obtainable from the commercially availablereinforced products. An alternative process of forming moulded articlesby melting and homogenising short lengths, that is lengths between 2 and100 mm, of the reinforced products of the invention is by calendering.For example, a sheet product can be made in this manner.

The products suitable for injection moulding may be used directly or maybe blended with pellets of other thermoplastics products. These otherproducts may be of the same polymer but having higher molecular weightor may be of a different polymer providing that the presence of thedifferent polymer does not adversely affect the overall balance ofproperties of the composition. The other products may be an unfilledpolymer or may contain a particulate or fibrous filler. Blends withmaterials containing the conventionally produced reinforced mouldingpowders, that is moulding powders with reinforcing fibres up to about0.25 mm long are particularly suitable because the overall reinforcingfibre content of the blend can be kept high to produce maximum strengtheven though the shorter reinforcing fibres do not contribute soeffectively as the long fibres present from the product of the presentinvention.

The chopped form of the continuous pultrusion is also very useful as afeedstock for the method described in copending British PatentApplication No. 8101822 in which a fibre-reinforced shaped article isproduced by extruding a composition comprising a settable fluid as acarrier for fibres at least 5 mm in length through a die so thatrelaxation of the fibres causes the extrudate to expand to form an openfibrous structure containing randomly dispersed fibre as the extrudateleaves the die and compressing the porous structure produced whilst thecarrier is in a fluid condition into a shaped article.

By "settable" is meant that the fluid may be "set" into such a form thatit holds the fibre in the random orientation, which occurs on extrusion.Thus, for example, the settable fluid may be a molten thermoplasticsmaterial which is extruded in its molten state and then set by coolinguntil it freezes.

Preferably the expanded extrudate is extruded directly into a mouldchamber provided with means for compressing the porous extrudate into ashaped article and the extrudate is compressed into a shaped articlebefore the extrudate is caused or allowed to set.

The extrudate formed in the process contains randomly dispersed fibresso that the only orientation of fibres in the shaped article is thatwhich might arise as a result of the compression process.

The process can be used at high fibre loadings, that is in excess of 30%by volume of fibre. Little fibre breakage occurs in the process so thatshaped articles of exceptionally high strength measured in alldirections in the article can be obtained.

Pellets obtained by chopping the pultruded product of the presentinvention into lengths of at least 5 mm and preferably at least 10 mmare preferred. The upper limit is determined by the extent of theproblems encountered in feeding material to the extruder which will meltthe product. Lengths at least up to 50 mm can be employed although withlong lengths the amount of fibre breakage increases so that the benefitof long fibre length is partially eroded.

Although it is necessary to use the relatively low molecular weightpolymers, for example polymers having a melt viscosity below 30 Ns/m²and preferably below 10 Ns/m² to achieve adequate wetting of the rovingand although it is surprising that such a product has such high levelsof physical properties the invention does not exclude the subsequentprocessing step of increasing the molecular weight of the polymer in thecomposition by known techniques. Such techniques include solid phasepolymerisation in the case of condensation polymers, the use ofcross-linking agents or irradiation techniques. In the case ofincreasing the molecular weight using cross-linking agents it isnecessary to intimately mix these in the composition. This may only bepracticable if they are already present during the impregnation processbut in such cases care must be taken to ensure that they are notactivated before the wetting process is complete.

The invention is further illustrated with reference to the followingexamples.

EXAMPLE 1

Copolymers of polyethylene terephthalate in which 20% by weight of theterephthalic acid had been replaced by isophthalic acid and having theintrinsic viscosity values listed in Table 1 were used to preparepolymer melts in a bath at a temperature of approximately 290° C. Aglass roving containing 16000 individual filaments was pulled throughthe molten polymer over one spreader bar situated in the bath at a rateof 30 cm/minute giving a dwell time in the bath of 30 seconds. Theimpregnated roving was pulled through a 3 mm diameter die in the wall ofthe bath and then cooled.

The viscosity of the melt and the intrinsic viscosity of the polymerfeedstock and the polymer in the reinforced composition were measured.The extent of the wetting of the fibres and the void content wereassessed by comparing the weight of a completely wetted length ofimpregnated product with the same length of product with an unknownextent of wetting. The completely wetted control material is obtained byoperating the pultrusion process at a very slow rate with a lowviscosity melt so that a completely transparent product is obtained.Thus the completely wetted standard is taken to be a sample which istransparent and which has been prepared under conditions which optimisethe parameters favourable to wetting. The extent of wetting values givenin the table are derived from the relationship: ##EQU1## where the massper unit length of the transparent sample is M₀, the mass per unitlength of the glass is M₁ and the mass per unit length of the sample tobe assessed is M₂. The void content is given by subtracting thepercentage degree of wetting from 100%.

The strength of the product was assessed by measuring the force requiredto break a specimen of the 3 mm rod in flexure placed across a 64 mmspan.

The results obtained are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Intrinsic viscosity                                                                           Melt viscosity                                                                           Extent of                                          (dl/g)          during     fibre    Force at                                           After      pultrusion wetting                                                                              break                                   Feedstock                                                                              pultrusion (Ns/m.sup.2)                                                                             (%)    (N)                                     ______________________________________                                        0.18      0.15      0.2        100    129                                     0.4       0.35      1.8        100    158                                     0.43      0.38      3.0        100    151                                     0.45      0.4       6           96    143                                     0.49      0.44      15          86    154                                     0.60      0.5       30          70    122                                     ______________________________________                                    

EXAMPLE 2

The polymer used in Example 1 having an intrinsic viscosity of 0.45 dl/gwas evaluated over a range of melt temperatures and haul through rates.The results obtained are recorded below in Table 2.

                  TABLE 2                                                         ______________________________________                                                            Extent of                                                                     fibre wetting                                                                             Force at break                                Melt      Melt      (%)         (N)                                           temperature                                                                             viscosity Haul through rate of (cm/min)                             (°C.)                                                                            (Ns/m.sup.2 )                                                                           21     36   60  21   36    60                             ______________________________________                                        255       18         98     84  62  102   70   40                             275       10        100    100  90  121  114   70                             290        6        100    100  92  139  127   94                              310*      3         98     98  92  155  140   94                             ______________________________________                                         *Excessive degradation occurred at this temperature.                     

EXAMPLE 3

A PET homopolymer having a melt viscosity at 280° C. of 6 Ns/m² waspultruded as described in Example 1 using a glass fibre made up offilaments of diameter 17 μm at 280° C. using a single spreader bar and aline speed of 30 cm/minute to give a pultruded rod approximately 3 mmdiameter. The glass content of the product was varied by altering thenumber of strands in the rovings fed to the bath. The flexural modulusand force at break were determined as a function of glass content usinga 64 mm span.

                  TABLE 3                                                         ______________________________________                                        Weight % glass      Flexural                                                  By        By ash    Modulus  Force at break                                   calculation                                                                             content   (GN/m.sup.2)                                                                           (N)                                              ______________________________________                                        62        63        42(3)    217(19)                                          56                  37(2)    201(36)                                          52                  38(3)    204(16)                                          47                  27(5)    191(24)                                          43                  17(4)    112(25)                                          37        36        12(1)     92(25)                                          ______________________________________                                    

(Five determinations in each case, figure in parenthesis indicatesstandard deviation.)

These results indicate an approximate plateau in modulus and strength inthe region 50 to 65% glass (by weight).

EXAMPLE 4

Conventional grades of polypropylene have viscosities at low shear ratesin excess of 100 Ns/m² and are not conveniently processed by pultrusion.For example the melt viscosity of `Propathene` HF11, a polypropylenehomopolymer, is about 3000 Ns/m² at low shear at 280° C. or about 10000Ns/m² at 230° C. In order to make a polymer suitable for pultrusion`Propathene` HF11 was blended with 0.1% calcium stearate, 0.1% `Irganox`1010 and 0.5% `Luperco` 101XL (`Luperco` 101XL is an organic peroxidedispersed with calcium carbonate) so that degradation would occur. Thiscomposition was pultruded at 30 cm/minute using a single spreader attemperatures of 230° C. and 290° C. At 230° C. (melt viscosity 30 Ns/m²)wetting was poor. At 290° C. (melt viscosity 17 Ns/m²) the wetting wasmoderate.

EXAMPLE 5

A sample of `Victrex` polyethersulphone having an RV of 0.3 waspultruded with the glass fibre used in Example 3 at 405° C. at 21cm/minute using a single spreader bar (having a melt viscosity of 30Ns/m²) giving a moderately wetted extrudate. At lower temperature, wherethe viscosity was higher, the sample was poorly wetted.

EXAMPLE 6

The wetting of the rovings is clearly influenced by the number ofspreader bars, and for the same operating conditions an increase in linespeed can be effected by increasing the number of spreaders for anydegree of wetting.

The glass fibre used in Example 3 was pultruded at 280° C. with PEThomopolymer using a single spreader and a speed of 20 cm/minute to givea totally wetted product (transparent). The dwell time in the bath underthese conditions was about 30 seconds. The use of three spreadersallowed an increase in line speed to 120 cm/minute for a transparentwell-wetted pultrusion. The dwell time under these conditions was about10 seconds.

EXAMPLE 7

A number of polymers were used according to the general procedure ofExample 1 to produce pultruded sections from a glass roving containing16000 filaments. The roving was pulled through molten polymer over onespreader bar at a rate of 15 cm/minute to give a product containingabout 65% by weight of glass in each case. The polymers used, the melttemperatures employed, the melt viscosities at those temperatures andthe properties obtained are detailed in Table 4.

                                      TABLE 4                                     __________________________________________________________________________              Processing Conditions                                                                     Physical Properties                                               Melt   Melt Flexural                                                                           Flexural                                                                           Interlaminar                                            Temperature                                                                          Viscosity                                                                          Modulus                                                                            Strength                                                                           Shear Strength                                Polymer Type                                                                            (°C.)                                                                         (Ns/m.sup.2)                                                                       (GN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (MN/m.sup.2)                                  __________________________________________________________________________    Poly(ethylene                                                                           290    6    31   550  42                                            terephthalate)                                                                Nylon 66   320*  30   28   600  --                                            Poly(methyl                                                                             250    8    30   550  --                                            methacrylate)                                                                 Polypropylene                                                                           260    5    30   350  10                                            Polyetherether-                                                                         400    20   30   650  45                                            ketone                                                                        Polyethersulphone                                                                       350    8    30   500  --                                            Polyphenylene                                                                           320    5    40   750  23                                            sulphide                                                                      __________________________________________________________________________     * = Some degradation occurring.                                          

In the case of the polyethylene terephthalate the pull through speed wasincreased above 15 cm/mm to examine the effect of void content onphysical properties. Table 5 below records properties measured on the 3mm diameter rod produced. These indicate that a void content of lessthan about 5% gives superior properties.

                  TABLE 5                                                         ______________________________________                                        Void  Flexural Flexural Interlaminar                                                                            Flexural Modulus                            content                                                                             Modulus  Strength Shear Strength                                                                          as percentage of                            (%)   (GN/m.sup.2)                                                                           (MN/m.sup.2)                                                                           (MN/m.sup.2)                                                                            calculated value                            ______________________________________                                        0.2   31       550      42        91                                          5     31       570      42        91                                          6     28       550      41        90                                          10    24       480      32        82                                          13    23       440      36        74                                          15    20       330      29        71                                          ______________________________________                                    

EXAMPLE 8

A sample of carbon fibre-reinforced polyetherketone was prepared bypulling a carbon fibre tape containing 6000 individual filaments througha bath of molten polyetherketone at a temperature of 400° C. at a speedof 25 cm/minute. A product having a flexural modulus of 80 GN/M², abreaking stress of 1200 MN/m² and an interlaminar shear stress of 70MN/m² was obtained.

EXAMPLE 9

This example illustrates how the mechanical properties of pultrusionsvary with the volume fraction of fibre and with the resin type. Thesamples are compared at fixed volume concentration. The low flexuralstrength of the composites based on polypropylene is a reflection of thetendency for less stiff resins to fail in a compressive mode.Polypropylene resin has a modulus of about 1 GN/m² while polyethyleneterephthalate has a modulus of about 2 GN/m². The pultrusions wereproduced according to the general procedure of Example 1 with resins ofthe preferred viscosity level, about 3 Ns/m².

                  TABLE 6                                                         ______________________________________                                                                            Interlaminar                                         Volume   Flexural Flexural                                                                             shear                                                fraction Modulus  strength                                                                             strength                                  Resin      glass %  (GN/m.sup.2)                                                                           (MN/m.sup.2)                                                                         (MN/m.sup.2)                              ______________________________________                                        PET        40       21       630                                                         50       31       690    26                                                   60       38       800                                              Polypropylene                                                                            40       26       340                                                         50       33       340    10                                                   60       38       310                                              ______________________________________                                    

This Example indicates a clear preference for a resin of high modulusfor applications where high compressive strength is required.

EXAMPLE 10

A sample of 64% by weight glass in PET was pultruded to form a tape 6 mmwide by 1.4 mm thick. This tape was remelted and wound under tensiononto a former 45 mm in diameter and consolidated on the former and thenallowed to cool. After cooling the former was withdrawn leaving afilament wound tube. Tubes of varying thickness up to 4 mm were wound inthis way.

EXAMPLE 11

Samples of 3 mm diameter uniaxially oriented pultrusion based on PETcontaining 64% by weight glass were remelted and twisted so that thefibres were given a helical form. These twisted rods were tested inflexure and the stiffness breaking force and total work to failure weremeasured. The total work of failure was determined as the area under theforce deformation curve up to failure and for convenience is presentedhere as a function of the area under the untwisted control sample.

                  TABLE 7                                                         ______________________________________                                                  Flexural    Breaking  Comparative                                   Angle of  Modulus     Force     total work                                    twist     (GN/m.sup.2)                                                                              (N)       to failure                                    ______________________________________                                        0°                                                                           control 36          135     1                                           11°    33          118     1.3                                         23°    24           85     1.6                                         ______________________________________                                    

It is noted that at 11° there is only a 10% reduction in stiffness andbreaking force whereas total work to failure is increased by 30% givingan improved balance of properties. At 23° the stiffness and strengthsare both reduced by about 60% and the work of failure is only increasedby 60%. This indicates an optimum twist of the order of 11°.

Thermoplastic pultrusions are more suitable than thermoset pultrusionsfor taking advantage of this energy absorbing mechanism because of theease with which they can be post formed.

EXAMPLE 12

Pultrusions 3 mm in diameter and containing 50% by volume glass fibre inPET were melted at 280° C. and than braided together. The braidedproduct was less stiff then uniaxially aligned material but absorbedmore energy in testing for impact failure.

EXAMPLE 13

Flat tape approximately 1.4 mm thick and 6 mm wide formed of 50% byvolume (64% by weight) glass fibre in PET were woven together in an opentabby weave. Four layers of that weave were stacked together andcompression moulded at 280° C. into a sheet 3 mm thick. The sheet hadthe following properties:

    ______________________________________                                        Flexural modulus (maximum)*                                                                           15 GN/m.sup.2                                         Impact energy                                                                 initiation               7 J                                                  failure                 25 J                                                  ______________________________________                                         *somewhat lower values would be expected at an angle of 45° to the     natural orientation of the weave.                                        

EXAMPLE 14

Examples of various pultrusions were placed in the moulds ofconventional injection mouldings and compatible polymer was mouldedround them. The mouldings had enhanced stiffness and strength.

Thermoplastic pultrusions are especially suitable for reinforcingmouldings in this manner because they can be made with a polymer whichis entirely compatible with the polymer to be moulded around thereinforcement.

EXAMPLE 15

A rod of 65% by weight glass fibre in PET was chopped to 1 cm lengthsand diluted on a 50/50 basis with normally compounded materialcontaining 30% by weight of short glass fibre in PET. This mixture wasinjection moulded using normal technology to give ASTM bars having thefollowing properties in comparison with normally compounded materialcontaining 50% by weight of glass fibre in PET.

                  TABLE 8                                                         ______________________________________                                                    Blend containing                                                                           Normally                                                         pultruded material                                                                         compounded                                           ______________________________________                                        Flexural Modulus                                                                            16.1           14.4                                             Impact Energy 270 J/m        120 J/m                                          notched Izod test                                                             Glass content (wt %)                                                                        47%            45%                                              ______________________________________                                    

Inspection of ashed sections of the moulding revealed that most of thelong fibres had been retained through the moulding operations. Thisunexpected property is believed to result from the low void content orhigh degree of fibre wetted by polymer in the chopped pultrudedmaterial.

EXAMPLE 16

Various Examples of pultruded materials including PET with 60% by weightof glass fibre and PEEK containing 60% by weight of carbon fibre werechopped to 1 cm length and moulded using the method described in BritishPatent Application No. 8101822 in which an expanded reinforced materialis produced by extrusion through a die of short length, preferably ofzero length, and is subsequently compression moulded to give threedimensional shaped articles containing 60% by weight of long fibre.

Pultruded material is especially suitable for this application becausethe high level of wetting obtained effectively protects the fibres andreduces the attrition between them which causes fibre breakdown.

EXAMPLE 17

The procedure of Example 1 was followed to produce a tape formed in acooled sizing die approximately 1.4 mm thick by 6 mm wide at line speedsof about 0.2 m/minute using PET having a melt viscosity at 280° C. of 3Ns/m².

Not all commercial glass fibres are ideal for pultrusion withthermoplastics. The most important difference is the size system used.Several commercially available grades were compared together with astudy of the effect of crystallinity. As produced the pultrusions wereamorphous but they were readily crystallised by heating to 150° C. Inthe following table all samples of different glass are compared at thesame weight fraction of 64% by weight of glass fibre.

                                      TABLE 9                                     __________________________________________________________________________           Properties in flexure at 23° C.                                        Amorphous resin                                                                              Crystalline resin                                              Modulus                                                                            Strength                                                                           ILSS Modulus                                                                            Strength                                                                           ILSS                                          Glass Fibre                                                                          (GN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (GN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (MN/m.sup.2)                                  __________________________________________________________________________    A      25   520  33   32   510  24                                            B      31   690  26                                                           C      21   300  17   29   310  14                                            D      30   771  41   35   733  17                                            E      28   420  40   34   523  36                                            F      27   533  38   30   520  30                                            __________________________________________________________________________

The crystalline form, offering higher stiffness, is to be preferred formany applications but it is important that a high value of interlaminarshear stress (ILSS), preferably greater than 20 MN/m², is retained.

EXAMPLE 18

High performance composites frequently need to be capable of performinga service at high temperature. Using 64% by weight of the glass E usedin Example 17 in PET, the following properties were determined atelevated temperature for crystalline pultrusions.

                  TABLE 10                                                        ______________________________________                                                 Flexural Properties                                                  Temperature                                                                              Modulus     Strength ILSS                                          (°C.)                                                                             (GN/m.sup.2)                                                                              (MN/m.sup.2)                                                                           (MN/m.sup.2)                                  ______________________________________                                         23        34          523      36                                             50        27          433      30                                             70        27          411      32                                            100        26          348      27                                            150        24          189      24                                            200        23          180      19                                            ______________________________________                                    

EXAMPLE 19

Hot water is a common aggressive environment in which composites arerequired to retain their properties. Samples based on 64% by weight ofthe glass fibre E used in Example 17 pultruded with PET and immersed ina water bath at 95° C. for varying times. Samples were tested bothamorphous and crystalline. Properties deteriorated with time,interlaminar shear strength (ILSS) being the most sensitive property.

                  TABLE 11                                                        ______________________________________                                        Immersion time  ILSS (MN/m.sup.2)                                             (hours)         Amorphous Crystalline                                         ______________________________________                                        0               40        34                                                  0.75            39        30                                                  4               35        31                                                  24              24        28                                                  48              32                                                            70              25        22                                                  94              19        22                                                  112             22                                                            ______________________________________                                    

In some other glass systems the interlaminar shear strength deterioratedto less than 10 MN/m² after 4 hours exposure.

EXAMPLE 20

Resistance to fatigue is an important factor in the service propertiesof composite materials. Samples of well wetted pultrusion were preparedbased on 64% by weight of the glass fibre E used in Example 17 in PET. Asample was tested in flexure to study the stress/strain relationship at23°.

                  TABLE 12                                                        ______________________________________                                        Stress       Strain    Stress/strain                                          (MN/m.sup.2) (%)       (GN/m.sup.2)                                           ______________________________________                                        150          0.48      31                                                     228          0.74      31                                                     300          0.96      31                                                     377          1.24      30                                                     464          1.60      29                                                     532          1.96      27                                                     584          2.30 broken                                                                             25                                                     ______________________________________                                    

The samples has a linear elastic limit at 1% strain. Samples were flexedin three point bending using a span of 70 mm at a rate of one cycleevery two seconds. The number of cycles was noted for significant damage(judged by a whitening of the pultrusion) to be induced.

                  TABLE 13                                                        ______________________________________                                        Strain in fatigue test                                                        (%)            No. of cycles to damage                                        ______________________________________                                        0.86           >1,100,000                                                     1.03           50,000                                                         1.20              22                                                          1.37              14                                                          1.54              8                                                           1.70              1                                                           ______________________________________                                    

Samples were strained at 0.1% strain and their properties were evaluatedafter different histories.

                  TABLE 14                                                        ______________________________________                                                   Flexural Properties                                                             Modulus     Strength ILSS                                        No. of cycles                                                                              (GN/m.sup.2)                                                                              (MN/m.sup.2)                                                                           (MN/m.sup.2)                                ______________________________________                                            0        30          530      38                                          174,000      29          470      38                                          600,000      28          460      33                                          773,000      30          500      36                                          standard deviation                                                                          3           40       3                                          ______________________________________                                    

The tests included evaluating the sample with the surface which had beenunder tension during the fatigue history in both compression andtension. No difference were observed in these two modes.

The properties of the pultrusion were not affected by this fatiguehistory.

EXAMPLE 21

Samples of tape approximately 1.4 mm thick by 6 mm wide were preparedbased on the glass fibres used in Example 17 in PET. The glass contentwas varied and in all cases the pultrusions were translucent.

                  TABLE 15                                                        ______________________________________                                                Flexural Properties                                                             Modulus      Strength ILSS                                          Wt % glass                                                                              (GN/m.sup.2) (MN/m.sup.2)                                                                           (MN/m.sup.2)                                  ______________________________________                                        46        17           250      33                                            64        30           770      41                                            73        35           950      40                                            78        40           1040     44                                            ______________________________________                                    

EXAMPLE 22

High line speeds are highly desirable for economic production.Pultrusions were formed containing 69% by weight of the glass fibre Dused in Example 17 in PET by drawing the pultrusion through a melt bathcontaining five spreading bars. Well wetted pultrusions were obtained atthe following speeds and their properties were measured in flexure.

                  TABLE 16                                                        ______________________________________                                        Line speed                                                                              Modulus      Strength ILSS                                          (m/minute)                                                                              (GN/m.sup.2) (MN/m.sup.2)                                                                           (MN/m.sup.2)                                  ______________________________________                                        0.48      30           640      41                                            0.78      29           670      40                                            2.76      31           640      --                                            ______________________________________                                    

EXAMPLE 23

Pultrusions were made from the glass fibre E used in Example 17 in PETusing a single spreader at 280° C. The viscosity of the resin wasvaried. With very low viscosity resin some resin was squeezed from thepultrusion at the shaping stage where it was compressed into a tape 6 mmwide by 1.4 mm thick. The line speed was set at 0.2 m/minute. Thepultrusions were tested in flexure in both the amorphous and thecrystalline form. The crystalline form was obtained by heating thesample briefly to 150° C.

                                      TABLE 17                                    __________________________________________________________________________    Resin melt                                                                    viscosity                                                                           Glass                                                                             Amorphous      Crystalline                                          at 280° C.                                                                   content                                                                           Modulus                                                                            Strength                                                                           ILSS Modulus                                                                            Strength                                                                           ILSS                                       (Ns/m.sup.2)                                                                        (wt %)                                                                            (GN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (GN/m.sup.2)                                                                       (MN/m.sup.2)                                                                       (MN/m.sup.2)                               __________________________________________________________________________     0.01 73  37   670  21   21   180  --                                          0.1  75  36   690  --   22   350  --                                          3    64  28   550  41   29   550  31                                         20*   69  27   780  --   --   --   --                                         40    ← not possible to impregnate →                              __________________________________________________________________________     *poorly wetted                                                           

Samples of very low viscosity gave useful properties in the amorphousstate but when crystallized the properties deteriorated badly.

At high viscosity the glass was poorly wetted (so giving a low resinconcentration).

EXAMPLE 24

Tapes of the glass fibre E used in Example 17 were pultruded (in PET ofmelt viscosity 3 Ns/m² at 280° C.) over a single spreader to give wellwetted tapes 6 mm wide but of different thicknesses, obtained byincorporating different amounts of glass. Samples tested were amorphous.

                  TABLE 18                                                        ______________________________________                                                      Flexural Properties                                                                 Modulus   Strength                                                                             ILSS                                     Thickness                                                                              Glass content                                                                            (GN/m.sup.2)                                                                            (MN/m.sup.2)                                                                         (MN/m.sup.2)                             ______________________________________                                        1.4 mm   64         28        550    41                                       2.8 mm   61         27        520    39                                       4.3 mm   68         33        570    31                                       ______________________________________                                    

EXAMPLE 25

Glass fibres having different diameters were pultruded with PET. Thesamples tested amorphous had the following properties.

                  TABLE 19                                                        ______________________________________                                                    Flexural properties                                               Fiber     Glass   Modulus    Strength                                                                             ILSS                                      diameter  content (GN/m.sup.2)                                                                             (MN/m.sup.2)                                                                         (MN/m.sup.2)                              ______________________________________                                        12 μm  71      31         655    30                                        17 μm  71      33         609    32                                        17 μm  65      31         --     --                                        24 μm  62      26         472    24                                        ______________________________________                                    

EXAMPLE 26

Polyethersulphone having a melt viscosity of 8 Ns/m² at 350° C. was usedto impregnate the glass fibre E used in Example 17 using a singlespreader system at a line speed of 0.2 m/minute. The followingproperties were obtained.

                  TABLE 20                                                        ______________________________________                                                Flexural Properties                                                             Modulus      Strength ILSS                                          Wt % glass                                                                              (GN/m.sup.2) (MN/m.sup.2)                                                                           (MN/m.sup.2)                                  ______________________________________                                        59        28           460      35                                            68        33           560      30                                            ______________________________________                                    

EXAMPLE 27

PEEK having a melt viscosity of 30 Ns/m² at 380° C. was used toimpregnate carbon fibre in a single spreader pultrusion device at 0.2m/minute. A rod 3 mm diameter was formed containing 60% by weight ofcarbon fibre.

EXAMPLE 28

Blends were made of conventional glass filled PET (short fibrecompounded material prepared by extrusion compounding with PET of IV -0.75) and chopped 10 mm pultrusions (prepared according to Example 3).These blends were injection moulded to give discs 114 mm diameter and 3mm thick filled from a rectangular side gate 1.5 mm thick by 10 mm wide.These samples were subjected to impact in an instrumented falling weightimpact test and the failure energy noted.

                  TABLE 21                                                        ______________________________________                                             Wt %     Wt %              Failure                                            short    long    Total Wt %                                                                              energy                                                                              Standard                                Test fibers   fibers  fibers    J     deviation                               ______________________________________                                        1    30       --      30        4.8   (0.6)                                   2    45       --      45        5.3   (0.6)                                   3    22.5     15      37.5      7.3   (0.6)                                   4    15       30      45        8.7   (0.4)                                   5    7.5      45      52.5      8.8   (0.4)                                   6    --       60      60        8.7   (1.0)                                   ______________________________________                                    

All samples filled the mould with similar ease. This is because thepolymer used to prepared the pultrusion samples was of lower molecularweight than that used to prepare the short fibre compound and this lowermolecular polymer offset the increased resistance to flow due to thelong fibre.

The results clearly indicate increased failure energy for the long fibrefilled material despite the lower molecular weight of the polymer whichwould normally be expected to contribute to brittleness. Note especiallythe comparison between tests No. 2 and No. 4 and the same total weightpercentage of fibres.

We further noted that whereas the short fibre mouldings splintered onimpact allowing pieces of sharp plastic to fly off when more than halfthe weight fraction was of long fibres the mouldings failed in a safemanner all broken pieces remaining attached to the main part.

Ashing the mouldings after test revealed that much of the long fibreglass had retained most of its original length. Considerably more than50% by weight of the original long fibres in the mouldings were morethan 3 mm long.

The samples were also assessed for flexural modulus, anisotropy ratio,Izod impact strength and IV of the polymer in the mouldings. The valuesin the table below indicate reduced anisotropy and good notched impactrelative to short fibre products.

                  TABLE 22                                                        ______________________________________                                                  Disc                                                                Bar       flexural                                                            flexural  modulus  Aniso-  Izod impact                                        modulus   (GN/m.sup.2)                                                                           tropy   notched                                                                              unnotched                                   Test (GN/m.sup.2)                                                                           0°                                                                            90°                                                                        ratio (Jm.sup.-1)                                                                          (Jm.sup.-1)                                                                           IV                              ______________________________________                                        1    10.3      9.7   6.3 1.54   70    414     0.53                            2    16.5     14.4   8.9 1.62   90    590     0.524                           3    11.4     10.7   7.3 1.47  117    432     0.46                            4    14.1     10.9   8.2 1.33  172    326     0.4                             5    15.1     10.3   7.7 1.34  195    312     0.34                            6    15.4     10.8   8.7 1.24  177    219     0.28                            ______________________________________                                    

EXAMPLE 29

14 tapes of continuous carbon fibres (supplied by Courtaulds Ltd anddesignated XAS carbon fibres), each containing 6000 individual filamentswere drawn at a rate of 25 cm/minute over a series of stationary guidebars to provide a band of width about 50 mm having a tension of about100 lbs. When the fibres had been guided into contiguous relationshipthey were pulled over a single fixed heated cylindrical bar of 12.5 mmdiameter. The temperature of the bar was maintained at about 380° C. Apowder of polyetheretherketone having a melt viscosity of 20 Ns/m² atthis temperature was fed to the nip formed between the carbon fibre bandand the fixed roller. The powder melted rapidly to provide a melt poolin the nip which impregnated the fibre band passing over the roller. Thestructure was passed over and under five further heated bars without theaddition of further polymer. A carbon fibre-reinforced sheet containing58% by volume of carbon fibre and having a thickness of 0.125 mm wasproduced. The product was found to have the following properties:

    ______________________________________                                        Flexural Modulus        130 GN/m.sup.2                                        Flexural Strength      1400 MN/m.sup.2                                        Interlaminar Shear Strength                                                                           90 MN/m.sup.2                                         ______________________________________                                    

EXAMPLE 30

The procedure of Example 29 was used with a polyethersulphone having amelt viscosity of 3 Ns/m² at 360° C. to produce a reinforced productcontaining 40% by volume of carbon fibre. The temperature of the rollerwas maintained at about 360° C. The product had a flexural modulus of 80GN/m² and a flexural strength of 700 MN/m².

EXAMPLE 31

The procedure of Example 29 was used with the commercially availablepolyethersulphone PES 200P (available from Imperial Chemical IndustriesPLC) having a viscosity of 800 Ns/m² at 360° C. The roller temperaturewas maintained at about 360° C. and a product with 44% by volume ofcarbon fibre was produced. The product had the following properties:

    ______________________________________                                        Flexural modulus        60 GN/m.sup.2                                         Flexural strength      500 MN/m.sup.2                                         Interlaminar shear strength                                                                           25 MN/m.sup.2                                         ______________________________________                                    

EXAMPLE 32

The general procedure of Example 29 was followed to produce animpregnated sheet using 14 tapes of continuous carbon fibres(Courtaulds' XAS, 6 K tow) and polyetherether ketone having a meltviscosity of 30 Ns/m² at 370° C. In the apparatus five cylindrical barseach of diameter 12.5 mm were heated to 380° C. The 14 tapes were drawnunder tension to give a band 50 mm wide passing into an adjustable nipformed by the first two bars with their longitudinal axes in ahorizontal plane. The band subsequently passed under and over threefurther heated bars also having their longitudinal axes in the samehorizontal plane. The use of the first two bars to form a nip enabled apolymer feed to be fed on both sides of the band. To avoid spillage ofpolymer two retaining metal sheets were placed in contact with the firsttwo heated bars, disposed along the length of the bars, to provide afeed trough. Polymer powder was fed to either side of the band passingthrough the first two heated bars. The powder melted rapidly forming amelt pool in the two nips between either side of the band and eachheated bar. The gap between the first two bars was adjusted so that whenthe haul-off was run at 0.5 m/minute the carbon fibre was coated withpolymer and the resulting impregnated tape contained approximately 60%by weight of carbon fibre and 40% by weight of polymer. Adjustments tothe fibre content were found to be achievable in several ways:

1 varying nip gap,

2 varying pretensioning,

3 varying the number of filaments fed to the nip,

4 varying the powder feed rate,

5 varying the temperature of the bars at the nip (with the resin used inthis example the temperature range preferred was not greater than 400°C. because of degradation and not less than 360° C. because of the onsetof crystallisation),

6 varying the haul-off rate.

The tape so formed appeared to be well wetted and was about 0.1 mmthick.

EXAMPLE 33

The tape described in Example 32 was cut to lengths of 150 mm andstacked in a matched die compression moulding tool. This tool was heatedto 380° C., in a conventional laboratory press, and compressed so thatthe moulding was subjected to a pressure of between 2 and 5×10⁶ N/m².The moulding was held at pressure for 10 minutes (approximately half ofwhich time was required for the mould and sample to reach equilibriumtemperature) and then cooled under pressure to 150° C. before removalfrom the press. The cooling stage took approximately 20 minutes. Themould was allowed to cool to ambient temperature and the moulding wasthen extracted.

Mouldings ranging in thickness from 0.5 mm (4 plies) to 4 mm (38 plies)were formed in this way. During the moulding operation a small amount ofpolymer was squeezed out of the mould as flash so that the mouldingscontained 62% by weight of carbon fibre by comparison with 60% by weightin the original tape.

The mouldings were then cut using a diamond wafering saw to givespecimens suitable for mechanical testing by flexural techniques. Thefollowing results were obtained:

    ______________________________________                                                    Specimen                                                          Property    span/depth ratio                                                                             Value                                              ______________________________________                                        Flexural modulus                                                                          70:1            130     GN/m.sup.2                                Flexural modulus                                                                          30:1            115 (6) GN/m.sup.2                                Flexural strength                                                                         30:1           1191 (55)                                                                              MN/m.sup.2                                Transverse flexural                                                                       5:1             98 (11) MN/m.sup.2                                strength                                                                      Interlaminar shear                                                                        5:1             81 (4)  MN/m.sup.2                                strength                                                                      ______________________________________                                         (figures in parentheses indicate standard deviation)                     

EXAMPLE 34

Using the same equipment as Example 32 some poorly wetted tapes wereproduced by starving some sections of the tape while flood feedingothers. The overall fibre content of the tape was the same as in Example4 but many loose fibres were apparent on the surface of the tapes whileother areas were rich in resin.

These tapes were stacked and moulded as described in Example 33 takingcare that poorly wetted regions of one tape were placed adjacent toresin rich areas in the next tape. Visual inspection of the mouldingsshowed that substantial unwetted areas remained and loose fibres couldeasily be pulled from the surface. The mechanical properties of thesemouldings was inferior to those noted in Example 33 and in particularthe interlaminar shear strength was variable and low, values of 10 MN/m²(by comparison with 81 for the well wetted sample) being common.

This Example illustrates that fibre wetting takes place primarily at theimpregnation stage and not during the secondary, moulding, stage. It is,however, believed that some wetting could be achieved by this secondarystage if higher pressures and longer dwell times were employed.

EXAMPLE 35

The tapes formed in Example 29 were split to give tapes approximately 15mm wide and these tapes were woven using a tabby weave (as described bythe Encyclopedia Britannica article on weaving) to give a sheetapproximately 150 mm square.

EXAMPLE 36

A single woven sheet as described in Example 35 was compression mouldedas described in Example 33 except that the moulding was simply carriedout between aluminium sheets without side-wall constraint. The mouldingwas a flat sheet 0.2 mm thick.

In a further experiment five woven sheets as described in Example 7 werelayed together such that each layer was oriented at ±45° to the layersabove and below it. This stack was compression moulded without side-wallconstraint to give a sheet 1 mm thick. A disc, 135 mm diameter, was cutfrom this sheet and the stiffness and strength of this disc weremeasured according to the techniques described by C.J. Hooley and S.Turner (Mechanical Testing of Plastics, Institute of MechanicalEngineers, June/July 1979, Automotive Engineer) using the disc flexuraltest and the automated falling weight impact test.

The flexural stiffness of the plate had a maximum value of 50 GN/m² anda minimum value of 36 GN/m².

The impact resistance of the sheet was as follows:

    ______________________________________                                        Initiation energy                                                                             1.7 (0.3) J                                                   Failure energy  6.6 (1.1) J                                                   ______________________________________                                         (standard deviation quoted in parentheses)                               

A parallel sided specimen cut along the line of maximum stiffness wasmeasured in conventional flexural testing to give:

    ______________________________________                                        Flexural modulus       51 GN/m.sup.2                                          Flexural strength     700 MN/m.sup.2                                          ______________________________________                                    

EXAMPLE 37

A disc 135 mm diameter and 1 mm thick was prepared according to theprocedure of Example 36 and subjected to nineteen impacts of 3J evenlydispersed over the surface of the disc. These impacts caused somedelamination but the damaged moulding remained coherent.

The damaged disc was then remoulded and then tested as described inExample 36 with the following results:

    ______________________________________                                                       Damaged and                                                                   remoulded                                                                              Virgin (Ex. 5)                                        ______________________________________                                        Flexural stiffness (max.)                                                                      51 GN/m.sup.2                                                                            50 GN/m.sup.2                                     Flexural stiffness (min.)                                                                      37 GN/m.sup.2                                                                            36 GN/m.sup.2                                     Impact initiation                                                                              1.9 (0.1) J                                                                              1.7 (0.3) J                                       Impact failure   6.5 (2.8) J                                                                              6.6 (1.1) J                                       ______________________________________                                         (standard deviation in parentheses)                                      

There is no significant difference in the results.

This Example demonstrates total reclaim of properties after partialdamages.

EXAMPLE 38

A damaged disc as prepared in Example 37 was broken through in impactfive times using the instrumented falling weight test. The damage waslocalised to an area not much greater than the cross-section of theimpactor and all broken parts remained attached to the main body of themoulding.

This broken moulding was then remoulded and impact tests were carriedout on it taking care to direct the new impact at the spot which hadpreviously been broken through giving the following results:

    ______________________________________                                        Initiation energy                                                                             1.8 (0.4) J                                                   Failure energy  4.6 (0.8) J                                                   ______________________________________                                         (standard deviation in parentheses)                                      

By comparison with the results of Examples 33 and 34 this shows that inthe worst possible case approximately 70% of the original strength canbe recorded.

EXAMPLE 39

A disc 135 mm in diameter and approximately 1 mm thick preparedaccording to Example 36 was heated to 380° C. and then placed in thefemale half of a cold hemispherical mould 200 mm diameter. The male halfof the mould was pressed down by hand and a section of a hemispherehaving a radius of curvature of 100 mm was formed. The section up to adiameter of ˜100 mm (subtended by a solid angle of ˜60° from the centreof the sphere of which it formed a part) conformed well to the doublecurvature but some crinkling occurred outside this area.

EXAMPLE 40

Woven sheets were prepared from 5 mm wide tape using a five shaft satinweave (as described in the Encyclopedia Britannica reference toweaving). In the dry state this weave gives excellent conformation todouble-curved surfaces without allowing holes to appear in the weave. Afive layer quasi-isotropic sheet was prepared and moulded as describedin Example 36. This 1 mm thick sheet was then heated to 380° C. andshaped against various cold surfaces including:

1 a right-angle,

2 a cylindrical surface having a radius of curvature of 25 mm,

3 a spherical surface having a radius of curvature of 15 mm.

In the case of 1 and 2 good conformation was obtained. For the doublecurvature good conformation was obtained up to a solid angle of 60°subtended from the centre of the sphere (this is similar to theexperience of Example 39 but at a tighter radius of curvature relativeto the thickness of the sheet).

Most large structures will require only gentle double curvature but fortight curvature narrower weaves and in particular satin weaves are to bepreferred to broad tabby weaves following general experience of theweaving industry.

EXAMPLE 41

A piece of material 40 mm square was woven (tabby weave) from tapes 2 mmwide and 0.1 mm thick. The formability of this material sheet wascompared with that of the broad tape weave described in Example 35. Thenarrower tapes allowed easier conformation to shape changes. Mouldedsheets formed from these two weaves appeared superficially similar inproperties.

It is likely that in order to make use of conventional weavingtechnology narrow tapes will be employed in practice.

EXAMPLE 42

An attempt was made to lay up the tapes formed in Example 32 to give amulti-layer composite where each layer had a different orientation.Because the tapes, as formed, have no "tack" at room temperature therewas a tendency for the layers to move relative to one another during theplacement and moulding operation so that the fibres were not oriented inthe designed configuration in the final moulding. This problem waspartly overcome by tacking the layers together locally with a solderingiron. When formed in this way the sheets had to be moulded withside-wall constraint to avoid the fibres flowing sideways and disruptingthe designed orientation pattern.

By contrast woven sheets were convenient and easy to handle and could bemoulded without side-wall constraint because the interlocking weaveitself prevents lateral movement of the fibres. The ability to form apreferred sheet without side-wall constraint is of especial advantagewhen considering the manufacture of continuous sheets by processes suchas double-band pressing.

EXAMPLE 43

Woven sheets according to Example 35 were layed up and moulded to givesheets of different thickness in which each layer was at ±45° to thelayer above and below it. The impact behaviour of these sheets wasdetermined using the instrumented falling weight impact test.

    ______________________________________                                                          Impact energy                                                          Thickness    Initiation                                                                             Failure                                      No. of layers                                                                            mm           J        J                                            ______________________________________                                        1          0.25         0.29     0.78                                         3          0.79         1.04     3.0                                          5          1.19         2.2      5.4                                          9          1.88         4.8      10.9                                         18         2.88         8.1      23.5                                         ______________________________________                                    

EXAMPLE 44

Following the procedure of Example 32 tapes were prepared from polyethersulphone `Victrex` 200P and carbon fibre (Courtaulds XAS, N. size). Thispolymer has a melt viscosity of 800 Ns/m² at 350° C. and 100 Ns/m² at400° C. The spreaders were controlled at about 370° to 380° C. and thehaul-off run at 0.2 m/minute. Because of the high viscosity of thisresin the tapes were less well wetted then those described in Example32. The resin content was increased slightly so that the final tapecontained 50% by weight of carbon fibre and 50% by weight of resin.

Samples were moulded as described in Example 33 to give uniaxiallyoriented sheets having the following properties:

    ______________________________________                                        Flexural modulus        60 GN/m.sup.2                                         Flexural strength      500 MN/m.sup.2                                         Transverse flexural strength                                                                          20 MN/m.sup.2                                         Interlaminar shear strength                                                                           26 MN/m.sup.2                                         ______________________________________                                    

Tapes were then woven and layed up and moulded according to Examples 35and 36 to give sheets approximately 1 mm thick having the followingproperties.

    ______________________________________                                        Flexural stiffness (maximum)                                                                      24 GN/m.sup.2                                             Flexural stiffness (minimum)                                                                      21 GN/m.sup.2                                             Impact energy (initiation)                                                                        2.9 (0.3) J                                               Impact energy (failure)                                                                           7.1 (0.3) J                                               ______________________________________                                         (standard deviation in parentheses)                                      

The broken through sheets were remoulded and retested taking care toimpact the samples on the same spot as the original impact damage.

The flexural stiffness of the remoulded sheet was 10% lower than that ofthe original sheet while the impact resistance was reduced to 60% of theoriginal value.

EXAMPLE 45

Polyethersulphone having a melt viscosity at 350° C. of 8 Ns/m² was usedto impregnate a tape of carbon fibre which had previously been sizedwith 5% by weight of polyethersulphone using a solution sizing process.This sample was impregnated by drawing it over four heated spreaders at350° C. at a speed of 0.2 m/minute. The final composite contained 47% byweight of carbon fibre. Samples were moulded according to Example 30 andtested giving the following results:

    ______________________________________                                        Flexural modulus        85 GN/m.sup.2                                         Flexural strength      680 MN/m.sup.2                                         Interlaminar shear strength                                                                           50 MN/m.sup.2                                         ______________________________________                                    

It will be noted that while this sample was prepared from polymer oflower molecular weight than that used in Example 44 the properties ofthe composite are superior.

EXAMPLE 46

Glass rovings were impregnated with polyethylene terephthalate (meltviscosity 3 Ns/m² at 270° C.) using the procedure described in Example32, but with the bars at 280° to 300° C. Up to 80% by weight of glassfibre could be satisfactorily incorporated to give good wetting. At 60%by weight of glass, line speeds of 5 m/minute were readily attained fortapes 0.1 mm thick.

EXAMPLE 47

Glass rovings were impregnated with polypropylene, having a meltviscosity at 270° C. of 10 Ms/m², using the same equipment as Example 32except that the bars were maintained at 270° C. At 50% by weight ofglass fibre a very well wetted tape 0.1 mm thick was obtained which wasespecially useful for overwrapping pipe and other sections made frompolypropylene.

EXAMPLE 48

Carbon fibre (`Celion` 6K and 3K tows) were impregnated with athermotropic polyester containing residues of hydroxynaphthoic acid,terephthalic acid and hydroquinone having a melt viscosity at 320° C. of7 Ns/m². The equipment was the same as described in Example 32 exceptthat the bars were maintained at 320° C. A tape 0.1 mm thick containing62% by weight of carbon fibre was of good appearance.

EXAMPLE 49

Various pieces of scrap material produced from working Examples 32 to 38including some material with excess resin were broken up and fed to aconventional screw extruder and compounded to form granules. Thegranules contained carbon fibres which were up to 0.25 mm long. Thesegranules were injection moulded using conventional moulding technologyunder normal operating conditions for filled PEEK. The mouldings had thefollowing properties by comparison with the best available commercialgrade of carbon fibre filled PEEK prepared by conventional compoundingoperations:

    ______________________________________                                                   Compounded scrap                                                                           Best commercial                                                  scrap        grade                                                 ______________________________________                                        Wt % carbon fiber                                                                           55             30                                               Modulus       32 GN/m.sup.2  13 GN/m.sup.2                                    Tensile strength                                                                           250 MN/m.sup.2 190 MN/m.sup.2                                    Surface quality                                                                            Good           Good                                              ______________________________________                                    

This Example illustrates that the product of this invention is capableof being converted to a product for conventional processing which is insome way superior to that available by present technology. Also thescrap from various long fibre operations such as sheet preparation,lamination, filament winding etc can be reclaimed to give a highperformance material. The characteristic of reclaimability is of greatsignificance when working with expensive raw materials such as carbonfibre.

EXAMPLE 50

The optimum tensions in rovings when operating according to the methodof Example 29 were determined by measuring the tension beforeimpregnation and at the haul-off stage in an individual rovingcontaining 6000 filaments (14 rovings were used in Example 29 and theoperating tension will in practice be 14 times the values given below).The values quoted below were judged to be the minimum operation tension(Case 1) and the maximum operating tension (Case 2) for the particularrovings, polymer type and equipment used. With tension values belowthose of Case 1 there was a tendency for misalignment of fibres andsplits in the tape produced. With tension values above those of Case 2increased fibre attrition was observed with loose fibres accumulating onthe band. For a different set of conditions (rovings, polymer type etcthe values obtained will be different but can be readily optimised toproduce a good quality product.

    ______________________________________                                               Tension before impregnation                                                                   Haul-off tension                                       ______________________________________                                        Case 1   0.14 kg           2.4 kg                                             Case 2   0.37 kg           3.8 kg                                             ______________________________________                                    

We claim:
 1. A molded article formed from a fibre reinforcedthermoplastic composition in a process which includes the step ofmelting and homogenizing a composition containing at least 30% by weightof fiber reinforced pellets between 2 mm and 100 mm long which pelletshave filaments extending the length of the pellet, characterized in thatthe molded article contains reinforcing filaments in the form ofindividual filaments and at least 50% by weight of the filaments in thepellets being present in the molded article at a length of greater than2 mm, the pellets having been cut from a structure of continuous,parallel, aligned, reinforcing filaments which have been wetted by amolten thermoplastic polymer in a melt pultrusion process.
 2. A moldedarticle according to claim 1 in which the molten thermoplastic has amelt viscosity of at least 30 Ns/m₂.
 3. A molded article according toclaim 1 in which the fibre reinforced pellets contain at least 30% byvolume of reinforcing filaments.